When tires become worn, they may be restored with new tread bands or tread layers during a retread process. Retreading is a restoration or re-manufacturing process that not only extends the service life of the tires, but also is significantly less expensive than manufacturing new tires. Since recycling and retreading are key for reducing costs and energy inherent in the manufacturing of tire casings, an effective retread necessitates a tire casing with good structural integrity (i.e., without internal anomalies or irregularities).
Prior and/or subsequent to retreading of a truck tire casing, a non-destructive testing (NDT) method may be used to detect and locate internal anomalies. Such anomalies may include, but are not limited to, cracks, voids, delaminated layers and/or foreign material. Numerous attempts have been made using advanced NDT techniques, and several types of inspection procedures have been employed and commercialized by the tire remanufacturing industry. For example, industrial radiography with X-ray is a real-time method for tire inspection. This inspection method provides high sensitivity to anomalies such as foreign materials and porosity (e.g., due to variations in radiation intensity of the X-rays penetrating through different materials). Smaller bonding irregularities, however, may be difficult to detect since the penetration capability of the X-rays is dependent on material density. Further inspection procedures are therefore required to investigate those conditions which are not detectable by X-rays.
Another known inspection method utilizes ultrasound vibration at higher frequencies (usually 1-10 MHz used in the tire industry) that attenuates much faster in the air than audible sound. The ultrasonic method can be used to examine abnormal cord spacing, belt anomalies or changes in the wall thickness of a tire. Automation of this testing procedure is complicated by the need for a coupling medium between the transducer and the object surface. In addition, the ultrasonic method requires extensive training and experience to interpret the data, therefore inhibiting the application of this inspection method in high-yield inspection environments.
Shearography, which is widely used for tire casing inspection, can detect various types of anomalies such as voids and de-laminations in belts and sidewalls. Using shearography, a tire is first scanned by a laser light placed in the center of the tire casing under normal atmospheric pressure to obtain a baseline photograph. The tire casing is placed in a vacuum. If there is an irregularity such as an air filled void, the low pressure around the casing causes the air trapped in the void to expand. A “stressed” photograph is obtained by scanning the tire casing under vacuum, which photograph is compared with the baseline photograph to produce a fringe pattern. An internal variance usually induces strain concentrations under stress that can cause differences between the two images and be translated into an anomaly in the fringe pattern. Shearography therefore correlates internal anomalies with the variances in displacement gradients. Interpretation of shearograms often requires a skilled operator, and shearography devices may require large fiscal and temporal expenditures, particularly when large batches of tires require examination.
Therefore, a reliable and cost-effective NDT technique is demanded that accurately determines the internal anomalies in a tire casing and does so predictably in a variety of production environments.